Background

Envenomation by some rattlesnakes, such as the Mojave rattlesnake (formerly Mohave rattlesnake) (Crotalus scutulatus), may cause a different clinical presentation than that generally encountered after most rattlesnake bites. In addition, other species, such as the Southern Pacific rattlesnake Crotalus oreganus helleri, (formerly Crotalus viridis helleri), may cause signs and symptoms consistent with typical rattlesnake envenomation combined with signs and symptoms similar to Mojave rattlesnake envenomation.
[1] (See Rattlesnake Envenomation for a more complete discussion of typical rattlesnake envenomation.)

Mojave rattlesnakes inhabit desert areas of the southwestern United States and central Mexico. Specimens with type A venom, which cause a different pattern of injury than other rattlesnakes, have been reported in southern California, Nevada, Utah, Arizona, Texas, and New Mexico.
[2, 3, 4, 5] Populations with venom B and intergrades of types A and B venom have been found in south-central Arizona, around Phoenix and Tucson.
[6]

The Mojave rattlesnake may be difficult to distinguish from the western diamondback rattlesnake (Crotalus atrox), which inhabits an overlapping geographical range. Some Mojave rattlesnakes are greenish, but they may have a similar color as western diamondbacks. In the Mojave rattlesnake, the diamond pattern fades into bands along the caudal third of the back, whereas the diamonds continue to the tail in the western diamondback.

The Mojave rattlesnake's white rings encircling the tail are much wider than the narrow black rings, whereas western diamondbacks have much more predominant black rings. The postocular stripe extends posteriorly above the mouth in the Mojave but intersects the corner of the mouth in the western diamondback. In Mojave rattlesnakes, supraocular scales are separated by fewer than 4 scales at their closest point. In western diamondbacks, at least 4 scales (usually >4) separate the supraocular scales.

See the image below.

This is a juvenile Mojave rattlesnake (postmortem). Note that the diamondback pattern fades into bands along the latter part of the snake's dorsum. Photo by Sean Bush, MD.

Other rattlesnakes in the Mojave rattlesnake's range and niche are distinguishable by the absence of a dorsal diamond pattern with light margins, black and white tail rings, facial stripes, or by the same criteria used to distinguish Mojave rattlesnakes from western diamondbacks.

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Pathophysiology

Venom A populations of Mojave rattlesnakes possess Mojave toxin, which has been experimentally shown to induce neurotoxic effects. Mojave toxin or a similar toxin has been detected in the venom of other rattlesnake species. This toxin impairs presynaptic acetylcholine release. Mojave toxin may cause severe neurologic effects clinically, although this presentation has been reported only a few times in the literature. Envenomation by several other species of rattlesnakes has been reported to cause serious neurologic signs and symptoms (eg, severe motor weakness, respiratory difficulty).

Venom A Mojave rattlesnakes cause less local injury and less hemorrhagic/proteolytic effects than other rattlesnakes. In contrast, venom B specimens cause local, proteolytic, and hemorrhagic effects typical of other rattlesnakes. Severe rhabdomyolysis with myoglobinuric renal failure has been reported with Mojave rattlesnake envenomation.
[7, 8, 9] This article focuses mainly on envenomation by venom A populations of Mojave rattlesnakes.